Filtration-compression type filter press

ABSTRACT

A filtration-compression type filter press comprising a fixed end plate and a movable end plate and compression filter plates suitably disposed therebetween. Each compression filter plate comprises a base plate two flexible fluid-tight sheets applied to the front and rear surfaces of the base plate respectively and having uneven filter surfaces with a filter cloth piece applied thereto to define pressure chamber between the base plate and each of the sheets and to provide a filtration chamber between adjacent filter plates for receiving a prefilt. The compression filter plate has a pressure fluid feed channel communicating with the pressure chamber a filtrate outlet channel open to its filter surface, and a prefilt supply channel open to the filtration chamber. The filter plate is further provided with another pressure fluid feed channel open to its filter surface and having a bypass channel in communication with the first-mentioned pressure fluid feed channel and a pressure fluid feed line and a branch line are provided with valves.

This application is a continuation of application Ser. No. 964,165,filed Nov. 28, 1978, now U.S. Pat. No. 4,237,009 which in turn is acontinuation-in-part of application Ser. No. 857,757, filed Dec. 5,1977, now abandoned.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a filter press of the so-calledfiltration-compression type wherein a slurry is fed under pressure andfiltered by the feed pressure as a primary treatment and upon completionof the filtration the resulting filter cake remaining in each filterchamber is compressed by the expansion toward the associated filterchamber of the compression diaphragm which faces toward the filterchamber to effect further dehydration as a secondary treatment.

Among filter presses of this type is a well-known one wherein thesurface of the compression diaphragm facing a filter chamber is madeuneven for filtration and for compression, thereby simplifying theconstruction. My U.S. Pat. No. 3,988,242 discloses a filter pressincluding a similar compression filter diaphragm composed of twoflexible fluid-tight sheets covering the front and rear surfaces of abase plate. The sheets are connected together by a cylindrical portionextending through a central opening of the base plate and defining aslurry feed port. The portion defining the slurry feed port is renderedfluid tight by a simple construction so as to keep the pressure chamberbetween the base plate and the diaphragm completely out of communicationwith the filtration chamber provided between each two adjacent filterplates, whereby the functions of filtration and compression as well asthe safety of the filtr press are ensured. The compression diaphragm canbe fitted to the base plate only by passing one of the flexiblefluid-tight sheets in a rounded form through the base plate opening fromone side thereof to the other side. To facilitate this procedure, thebase plate opening is made larger than the outside diameter of thediaphragm cylindrical portion. To prevent the attendant displacement ofthe diaphragm relative to the base plate in a direction in parallel tothe planar faces of these elements, a ring is fitted in the base plateopening around the cylindrical portion. Since one of the sheets mustalso be passed through the ring to fit the ring around the cylindricalportion, the ring used comprises radially divided two segments tofacilitate this procedure.

The slurry feed port portion at the center of the filter plate issubjected to high pressure, for example, of about 40 kg/cm² or higherduring the feed of the prefilt, whereas the cylindrical portion of thediaphragm is retained in the base plate opening by the ring which ismerely fitted in the opening and which is therefore freely movablerelative to the opening axially thereof. Consequently high pressure,when acting on the port portion, deforms the diaphragm cylindricalportion, which in turn displaces the ring axially thereof, thusproducing increased resistance to the passage of the prefilt through thefeed port. The cylindrical portion is therefore further deformed bybeing subjected to the resulting abnormal increase of the pressure. Thediaphragm would then be forced into pressing contact with one end of thering projecting from the base plate opening or with the opening edge ofthe plate at the other end of the ring. The diaphragm, consequentlyfatigued, would bulge progressively markedly, creating still increasedresistance to the passage of the prefilt, hence a vicious cycle. As aresult, the filter press fails to function properly in a short period oftime. Even if it is desired to use the diaphragm for a prolonged periodof time, cracks will develop in the cylindrical portion or in itsneighboring portion relatively early to impair the fluid-tightseparation between the pressure chamber and the filtration chamber andrendering the filter press inoperative. When the ring is in the form ofdivided two segments, the displacement of the ring and the fatigue ofthe diaphragm attendant thereon occur more markedly.

An object of the present invention is to provide afiltration-compression type filter press as disclosed in my U.S. Pat.No. 3,988,242, in which the cylindrical portion of the compressionfilter diaphragm and the ring fitting in the base plate opening aroundthe cylindrical portion can be held to the base plate opening portionwithout necessitating a cumbersome fitting procedure, or impeding theflow of pressure fluid through the pressure chamber between the baseplate and the diaphragm, or limiting the space available for the bulgingof the diaphragm to ensure a satisfactory compression function, so thatthe cylindrical portion can be held in place with safety by the ringwhich is held engaged in place as if it were an intergral part of thebase plate, even when the diaphragm is subjected to high pressure, thefiltr press thus being made operable with high efficiency and highstability over a prolonged period of time.

Another object of this invention is to provide a construction wherebythe pressure fluid used for the compression of cake is adapted to expelthe filtrate after the compression for the recovery of the filtrate withimproved efficiency.

According to the present invention, each of compression filter platessuitably disposed between a fixed end plate and a movable end platecomprises a base plate having a central opening considerably larger thanthe slurry feed port, and a filtration-compression diaphragm in the formof two flexible fluid-tight sheets to be fitted to the front and rearsurfaces of the base plate and having uneven surfaces with a filtercloth piece attached thereto, the diaphragm being formed with acylindrical portion which connects the sheets and which extends throughthe opening in the base plate and defines the slurry feed port, thediaphragm being adapted to be mounted on the base plate by wadding oneof the flexible sheets, passing the wadded sheet through the opening inthe base plate and unfolding the sheet. To facilitate the fitting of thediaphragm on the base plate, the opening is made sufficiently largerthan the slurry feed port. Further to prevent the displacement of thediaphragm relative to the base plate due to the presence of a clearancein the base plate opening around the cylindrical portion, a displacementpreventing ring is provided in the opening around the cylindricalportion. The ring is engaged at its outer periphery with theopening-defining inner periphery of the base plate axially thereof,whereby the ring can be held against any displacement even whencomprising radially divided two segments for the ease of installation,thus acting as if it were an integral part of the base plate to hold thecylindrical portion with safety and protect the same from an abnormalexternal force and deformation. Consequently the cylindrical portion aswell as the neighboring portion of the diaphragm is less prone tofatigue or bulging, affording fluid-tightness over a prolonged period oftime and rendering the filter press operable with improved stability.Additionally the means for engaging the ring, being provided on theplate inner periphery, will in no way affect the diaphragm, permittingthe pressure fluid to reach the cylindrical portion to freely bulge thediaphragm and thereby achieving still improved compression efficiency.The ring is held engaged in place by the fitting engagement of anannular groove formed in one of the ring outer periphery and theopening-defining inner periphery of the plate with an annular projectionformed on the other. The ring may be made of metal or any other materialhaving sufficient rigidity for the protection of the cylindricalportion. Rubbers are usable therefor. A rubber or like elastic ring, ifcut radially thereof only at one portion, is easily fittable around thecylindrical portion when opened at the cut portion. The ring is alsoeasily fittable into the opening by virtue of its elasticity. A metalring can be fitted around the cylindrical portion with ease whenradially divided into two. When such a ring is formed in its outerperiphery with an annular groove and divided into two along the groovetransversely of its axis, the ring is readily fittable into the baseplate opening.

The filter press of this invention further includes at least a pressurefluid feed channel communicating with a pressure chamber providedbetween the base plate and the diaphragm and a filtrate outlet channelin the diaphragm which opens to the diaphragm filter surface, wherebythe filter press is adapted to perform primary and secondary treatmentsor filtration and compression when the filter plates are clamped betweenthe fixed and movable end plates. The filter plate is further providedwith a pressure fluid bypass channel in communication with the feedchannel and open to the diaphragm filter surface, the arrangement beingsuch that the pressure fluid is valved to be admitted to the pressurechamber for the compression of cake and then bypassed to the filtersurface through the bypass channel for forcing the filtrate out throughthe filtrate outlet channel.

Therefore, the filtrate separated by filtration and compression andremaining in the filter surface grooves and outlet channel is forciblydischarged by the bypassed pressure fluid, so that when the filterplates are opened for cake discharge, there is no remaining filtratewhich would flow out. This ensures improved filtrate recoveryefficiency. When dehydrated cakes are required, there is no danger ofremaining filtrate flowing down to mix therewith and decreasing thedehydration efficiency.

Other features of the present invention will become more apparent fromthe following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a side view, partly in section, of a filtration-compressiontype filter press showing an embodiment of the invention;

FIG. 2 is an enlarged view of a portion of FIG. 1 clearly showing thepath of flow of pressure fluid;

FIG. 3 is a front view, partly broken away, of a compression filterplate shown in FIG. 1;

FIG. 4 is a front view of the filter plate of FIG. 3 with its filtercloth piece removed;

FIG. 5 is a front view, partly broken away, of an ordinary filter platein FIG. 1;

FIG. 6 is a perspective view showing a displacement preventing ring;

FIG. 7 is a view in section showing a modified displacement preventingring as incorporated in a filter plate;

FIG. 8 is a fragmentary enlarged view of FIG. 7; and

FIG. 9 is a front view showing the ring of FIGS. 7 and 8.

DETAILED DESCRIPTION

With reference to the embodiment shown in FIGS. 1 to 6, indicated at 1and 2 are machine frames which carry a pair of transversely spaced sidebars 3. A fixed end plate 4 is provided at one machine frame, and on theother machine frame 2 there is provided a movable end plate 5 which canbe advanced and retracted by a cylinder 6. The movable end plate 5, andfilter plates 7 of usual construction and improved compression filterplates 8, alternately disposed between the movable and fixed end plates5 and 4, are movably supported on the side bars 3 by handles 15projecting from their lateral surfaces. Each filter plate 7 of usualconstruction, as shown in FIGS. 1, 2 and 5, has uneven filter surfaces7a to which a filter cloth piece 9 is applied, whereas each compressionfilter plate 8, as shown in FIGS. 1 through 4, comprises a base plate 11having an opening 11a sufficiently larger than a slurry path 10, and afiltration compression diaphragm 13 applied to the front and rearsurfaces of the base plate to define pressure chambers 12 therebetween.The compression diaphragm 13 comprises flexible fluid-tight sheets 13bof rubber or the like having uneven surfaces 13a and applied to thefront and rear surfaces of the base plate 11, and a cylindrical portion13c extending through the opening 11a in the base plate 11, defining theslurry path 10 and interconnecting the sheets 13b. The diaphragm isfitted to the base plate 11 by folding one of the sheets 13b into two atthe slurry path 10, wadding opposite sides around the slurry path 10;passing the wadded sheet 13b through the opening 11a in the base platefrom one side thereof to the other side and unfolding the same. Thesheet 13b wadded can be passed through the opening 11a with greater easewhen bound as by a string. The compression filter plate 8 with thecompression diaphragm 13 mounted on the base plate 11 has a filter clothpiece 9 applied to the filter surfaces 13a defined by the sheets 13b.

The fixed and movable end plates 4 and 5 are disposed adjacent thecompression filter plates 8 and, although not shown, their opposedsurfaces are formed with filter surfaces similar to the filter surfaces7a of usual filter plate 7 and filter cloth pieces 9 are appliedthereto.

The filter plates 7 and 8, when clamped between the fixed end plate 4and the movable end plate 5 as a result of the advance of the latter,contact each other through the filter cloth pieces 9 as seen in FIG. 1to define filtration chambers 14 between the filter surfaces 7a and 13a.Each filtration chamber 14 adjoins a pressure chamber 12 separated by asheet 13b having a filter surface 13a.

The filter plates 7 and 8 have main filtrate outlet channels 17a whichcommunicate with filtrate outlet ports 16 formed in the fixed end plate4 at the lower opposite corners thereof when the filter plates arepressed into contact with each other. Each filter plate 7 has a filtrateoutlet sub-channel 17b which communicates with the left-hand sidefiltrate outlet channel 17a shown in FIG. 5 and which opens to thefilter surface 7a, while each filter plate 8 has a filtrate outletsub-channel 17b in the base plate 11 which communicates with theright-hand side filtrate outlet channel 17a shown in FIG. 3 and whichalso communicates with the filter surface 13a through holes 17c formedin the sheet 13. The filtrate outlet channel 17a of the plate 8 extendsthrough the base plate 11 and sheet 13b, opens to the marginal land 13dof the sheet 13b and is opposed to the outlet channel 17a which extendsthrough the filter plate 7 and opens to the marginal land 7b thereof.These outlet channels 17a in the filter plates 7 and 8 communicate witheach other when the lands 7b and 13d are pressed into contact with eachother. The outlet sub-channel 17b in the filter plate 8 communicateswith the outlet channel 17a within the base plate 11 and opens to aposition on the marginal land of the base plate 11 opposed to the holes17c in the sheets 13b and also opens to a corner of the filter surface13b surrounded with the marginal land 13d through the holes 17c. Theregion of the filter surface 13a where the holes are formed is opposedto an enlargement 7b' of the marginal land 7b surrounding the filtersurface 7a and is clamped between the enlargement and the base plate 11when the filter plates 7 and 8 are pressed into contact with each other,so that even if the compression diaphragm 13 is bulged as shown in theleft-hand side of FIG. 1 section by feeding pressure fluid to thepressure chamber 12, the region of the sheet 13b where the holes 17c areformed will not separate from the base plate 11. Therefore, the holes17c and outlet channels 17a, 17b do not spoil the fluid-tightness of thepressure chamber 12. The fixed end plate 4 has a filtrate outletsub-channel 17b which opens to its filter surface 4a, and main filtrateoutlet channels 17a which communicate directly with the filtrate outletports 16, the sub-channel 17b communicating with one of the channels 17aas in the case of the filter plates 7, while the movable end plate 5 hasa filtrate outlet sub-channel 17b which opens to its filter surface 5a,and main filtrate outlet channels 17a communicating with the filtrateoutlet ports 16 through the channels 17a in the plates 7, 8 and fixedend plate 4, the sub-channel 17b communicating with one of the channels17a in the movable end plate (FIGS. 1, and 3 through 5).

The filter plates 7, 8 and fixed and movable end plates 4, 5 havepressure fluid feed channels 19a which become continuous with each otherand communicate with a pressure fluid feed port 19 formed at one uppercorner of the fixed end plate 4 when these plates are pressed intocontact with each other, the channels 19a opening to the marginal landsof the respective plates. Each filter plate 8 is formed with a pressurefluid feed sub-channel 19b communicating with the associated feedchannel 19a within the base plate 11 and opening to the pressure chamber12. Main dual-purpose feed channels 21a which communicate with adual-purpose feed port 20 for washing water and pressure fluid formed atthe other upper corner of the fixed end plate 4 also open to themarginal lands of the plates 7, 8 and end plates 4, 5. The filter plates7, 8 are formed with dual-purpose feed sub-channels 21b whichcommunicate with the feed channels 21a and open to the filter surfaces7a and 13a. The dual-purpose feed channel 21b in each filter plate 8opens to the filter surface 13a through through-holes 21c formed at acorner of the filter surface 13a which will be pressed against anenlargement 7b' of the associated filter plate 7. A pressure fluid feedline 22 connected to the pressure fluid feed port 18 has its branch line22a connected to a washing liquid feed line 23 connected to thedual-purpose feed port 20. Main valves V22 and V23 are locateddownstream of the branching point of the branch line 22a of the line 22and downstream of a point at which the branch line is connected to thewashing liquid line 23, respectively. A valve V1 is providedintermediate between the ends of the branch line 22a and a valve V2 isprovided between the branching point of the branch line 22a and the feedport 18. A first discharge line 22b having a valve V3 therein isprovided between the branching point of the line 22a and the valve V1,while a second discharge line 23a having a valve V4 therein is connectedbetween the branching point of the washing liquid feed line 23 and thefeed port 20 (FIGS. 1 through 3). The numeral 24 in FIG. 1 designates aslurry feed inlet provided in the machine frame 1 and communicating withthe individual filter chambers 14 through the slurry paths 10 in thefixed end plate 4 and filter plates 7 and 8.

For filtration, the movable end plate 5 is advanced by the cylinder 6 toclamp the filter plates 7 and 8 between the movable and fixed end plates4 and 5 as seen in FIG. 1. In this state, a slurry is fed from the feedinlet 24 to the individual filter chamber 14. The slurry fed to thefilter chambers 14 is filtered under the feed pressure as it tends toflow through the filter cloth pieces 9 to the filter surfaces 7a and 13adefining the opposite lateral walls of the filtration chambers 14. Thefiltrate which has flowed to the filter surfaces 7a and 13a flows downalong the filter surfaces 7a and 13a to enter the filtrate outletsub-channels 17b which open to the filter surfaces 7a and 13a and it isfinally discharged through the filtrate outlet ports 16.

Upon the lapse of a predetermined period of filtration, the feed ofslurry is cut off and pressure fluid is fed to the feed port 18 throughthe pressure fluid feed line 22. In this case, all the valves except thevalves V22 and V2 remain closed, and the pressure fluid being fed flowsfrom the main pressure fluid feed channels 19a, which are now continuouswith each other, into the individual pressure fluid feed sub-channels19b and then into the individual pressure chambers 12, whereby thefiltration-compression diaphragm 13 of each filter plate 8 is bulgedfrom a first position shown at the right-hand side of the FIG. 1 sectionwherein the sheets 13b are in intimate contact with the base plate 11 toa second position shown at the left-hand side of said section whereinthey are separated from the base plate 11, thereby compressing the cake(not shown) for further dehydration resulting from said filtration andremaining in the filtration chambers 14. The resulting filtrate isdischarged via the same path as in the case of the preceding filtrate.

Upon completion of the dehydration, the valve V22 is closed whileleaving the valve V2 as it is, and the valve V1 is opened. As a result,the main pressure fluid feed channels 19a communicating with the washingliquid and pressure fluid feed channels 21a through portions of the feedlines 22 and 23 and the branch line 22a, so that the pressure fluid fedto the pressure chambers 12 flows back until it enters the dual-purposefeed channels 21a. Then it flows through the dual-purpose feedsub-channels 21b to the filter surfaces 7a and 13a of the filter plates7, 8, so that the filtrate remaining in the filtrate grooves in thefilter surfaces 7a and 13a is expelled through the filtrate outletchannels 17b, 17a. Thus the filtrate recovery efficiency is improved,and when cake is required there is no possibility that the filtrateremaining on the surfaces 7a and 13a from which it is to be recoveredwill mix in the cake to decrease the dehydration efficiency. Further,the pressure fluid for expelling the filtrate is mixed with the filtrateand discharged therewith. In this case, if compressed air is used as thepressure fluid, the resulting mixture is a gas-liquid one and the airwill spontaneously dissipate without any trouble.

When this filtrate expelling operation is not to be performed, only thevalves V2, V3 and V4 are opened after the dehydration of cake so as toallow the pressure fluid used for compression to be discharged throughthe first and second discharge lines 22b and 23a.

The series of operations described above, which constitutes one cycle offiltration, is repeated for filtration of successive batches of slurry.Each time the slurry is fed, the valves V3 and V4 are kept open for apredetermined period of time to enable the slurry being fed to releasethe air remaining in the filtration chambers 14. The timely operation ofthe individual valves may be automatically effected along withfiltration according to a predetermined program.

When filtration is repeated effected, the filter cloth pieces 9 willgradually clog, thus decreasing the filtration efficiency. In order toovercome this problem, the filter cloth pieces 9 are reversely washedperiodically or when required during the progress of the program. Thisreverse washing is effected by opening the valve V23 alone to feedwashing liquid from the feed port 20 through the washing liquid feedline 23. The washing liquid thus fed flows through the main dual-purposefeed channels 21a and then through the dual-purpose feed sub-channels21b to appear at the surfaces 7a and 13a. The slurry feed channels areopened as washing liquid outlet channels are closed so that the airtherein will not be forced out. The supplied washing liquid flowsthrough the filter cloth pieces 9 in a direction opposite to that forthe filtrate, i.e., from the filter surfaces 7a, 13a to the chambers 14.This reverse flow of washing liquid removes the remnant which sticks tothe filter cloth pieces 9 or with which the latter clog, and washes itout of the filter press via the slurry feed channels. Therefore, it goeswithout saying that the slurry feed channels will be selectivelyconnected to the slurry feed source and the washing liquid outlet port.

During each filtration, the filtration-compression diaphragms 13 areexpanded and contracted once, whereby the cylindrical portions 13c wouldbe fatigued to become incapable of sufficiently expanding andcontracting, interfacing with the flow of slurry. However, eachfiltration-compression diaphragm 13 is an open body comprising twosheets 13b connected around the slurry path 10 and is easy to form, andthe cylindrical portion 13c defining the slurry path 10 can be made tohave a thickness gradually increasing from the sheet 13b to avoid suchfatigue due to usual expansion. Since it is formed as a unit, no problemwill arise in its fluid-tightness around the slurry path 10. The filterpress is therefore properly operable for filtration and compression withhigh safety.

A displacement preventing ring 25 shown in FIG. 6 is fitted in the baseplate opening 11a around the cylindrical portion 13c of the diaphragm 13interconnecting the sheets 13b and providing the feed port 10 to preventthe displacement of the diaphragm 13 relative to the base plate 11. Thering 25 is made of material having suitable rigidity and elasticity suchas natural rubber. The ring is radially cut at one portion as seen inFIG. 6 and is easily fittable around the cylindrical portion 13c whenopened at the cut portion 25a by virtue of its elasticity. The ring hasan annular groove 25b in its outer periphery. When the cylindricalportion 13c has been fitted in the opening 11a with one sheet 13bpassing therethrough, the ring fits in the opening 11a, with an annularprojection 11b on the inner periphery defining the opening 11a infitting engagement with the annular groove 25b elastically, whereby thering 25 is held in place against axial displacement. The provision ofthe ring 25 will in no way make the diaphragm difficult to install inplace, therefore.

The ring 25, which per se is held engaged in the base plate opening 11aagainst displacement, retains the diaphragm cylindrical portion 13c inposition with high stability, thus obviating the displacement of thediaphragm 13 relative to the base plate 11 in a direction in parallel totheir planes and effectively preventing the axial deformation of thecylindrical portion 13c that would occur when the feed port portion 10is subjected to high pressure. Accordingly the cylindrical portion 13cas well as its neighboring portion will not be deformed excessively orpressed as against the edge of the opening-defining periphery and isless susceptible to fatigue or swelling, retaining high fluid-tightnessto separate the pressure chamber 12 from the filtration chamber 14 andperform repeated expansion and contraction in the usual manner. Thefilter press is therefore operable for filtration and compression withhigh safety over a prolonged period of time. Rings 25 of varyingthicknesses may be prepared for selective use, whereby the same baseplate 11 is usable with varying feed port sizes. The ring furtherpermits the pressure fluid to freely reach the cylindrical portion,rendering the diaphragm free to expand or contract even at the feed portportion 10 to effect dehydration with improved efficiency.

According to the embodiment shown in FIGS. 7 to 9, a metal ring 125 andan auxiliary metal ring 126 partially fitting to the front face of thering 125 are fitted in the opening 111a of a base plate 111 around acylindrical portion 113c of a compression filter diaphram 113 whichportion 113c interconnects the flexible fluid-tight sheet components113b of the diaphragm 113. The ring 125 has an inner periphery 125aconforming to the outer peripheral shape of the cylindrical portion113c. A projection 111b formed on the inner periphery of the base plate111 defining its opening 111a engages in an annular groove 125b providedin the outer periphery of the assembly of the rings 125 and 126. Therings 125 and 126 each comprise two radially divided segments 125c and126a which are fastened together by bolts 127 and are angularlydisplaced from each other by 90 degress. The bolts 127 passing throughthe ring 125 and screwed into threaded holes 126b of the ring 126 may bereversely screwed into the former ring 125. The rings 125 and 126 can befitted in place and fastened together with one sheet 113b passed throughthe opening 111a. Nuts are usable in place of the threaded holes 126bfor the bolts 127.

According to this embodiment, the rings 125 and 126, when installed inplace, hold the cylindrical portion 113c with improved effectiveness byvirtue of their high rigidity. The rings 125 and 126 are easilyinstallable because they are separate along the annular groove 125b andare each radially divided into two segments as mentioned above.

What is claimed is:
 1. A method for forced discharge of filtrate in afiltration-compression type filter press comprisingstopping slurry feedafter slurry is fed under pressure from a slurry feed inlet to afiltration chamber and filtered by a filter cloth piece and unevenfilter surface, feeding pressure fluid under pressure from a pressurefluid feed line to a pressure chamber of a compression filter plate,compressing cake in the filtration chamber by expanding flexiblefluid-tight sheets and discharging filtrate remaining in the cake,directing pressure fluid in the pressure chamber backward to adual-purpose feed channel of the compression filter plate through abranch line and a washing liquid feed line by closing a valve in theupper stream of the pressure fluid feed line alone and opening a valveof the branch line to direct it to a space between the filter clothpiece and the uneven filter surface of the sheet of the compressionfilter plate, whereby filtrate remaining in the filtrate groove of theuneven filter surface of the compression filter plate is forciblydischarged together with liquid contained in the cake through a filtrateoutlet channel by passing the filtrate through the filter cloth piece.2. In a filtration-compression type filter press including compressionfilter plates each comprising a base plate and two flexible fluid-tightsheets fitted to the front and rear surfaces of the base platerespectively and having uneven surfaces with a filter cloth pieceapplied to define a pressure chamber between the base plate and each ofthe sheets and a slurry path of the flexible sheets formed in a centralopening of the base plate, the compression filter plates being adaptedto be pressurized between a fixed end plate and a movable end plate todefine a filtration chamber in each space between the end plates, andthe compression filter plate, and the fixed end plate being providedwith a slurry feed inlet opened to the slurry path of the compressionfilter plate and a filtrate outlet port communicating with a filtrateoutlet channel of the compression filter plate, an apparatus for forceddischarge of filtrate comprising,a pressure fluid feed linecommunicating with a pressure fluid feed channel open to the pressurechamber of the compression filter plate connected to a pressure fluidfeed port of the fixed end plate, a branch line communicating with adual-purpose feed channel which is open to the uneven filter surface ofsaid sheet of the compression filter plate and communicates with a spacebetween said sheet and said filter cloth piece is bifurcated from thepressure fluid feed line, and a valve provided upstream of thebifurcation point of the branch line of the pressure fluid feed line anda valve provided downstream thereof.
 3. An apparatus for forceddischarge of filtrate as defined in claim 2 wherein the dual-purposefeed channel open to the uneven filter surface of the compression filterplate is connected to a washing liquid feed line open to the fixed endplate and serves as a pressure liquid feed line as well and the branchline from the pressure fluid feed line is connected to the downstream ofa valve of the washing liquid feed line open to said fixed end plate anda valve is provided in the branch line.
 4. An apparatus for forceddischarge of filtrate as defined in claim 2 wherein usual filter plateshaving uneven filter surfaces and a filter cloth piece applied theretoare disposed between said compression filter plates and centrally formedwith a slurry path, said usual filter plates have passages communicatingwith a filtrate outlet channel and the pressure fluid feed channel ofthe adjacent compression filter plate, and a dual-purpose feed channelcommunicating with the dual-purpose feed channel of the compressionfilter plate and open to said uneven filter surfaces and alsocommunicating with a space between said uneven filter surface and saidfilter cloth piece.